A conventional synchronization establishing method and user detection method will be described below. The conventional synchronization establishing method will be explained first. For example, the transmission side sends N phase-inverted signals (preambles) (see the top and the middle in FIG. 19). The reception side, on the other hand, detects inversion of the phase in a preamble, outputs a sync signal at this timing (see the bottom in FIG. 19), and demodulates data of a waveform to be modulated. Normally, to ensure accurate synchronization even in a channel with a lot of noise, the number N is often set equal to or more than 10.
The conventional user detection method will be explained next. Normally, the reception side checks if a received frame is a communication frame directed to the local device by using a user code to be transmitted following the preamble (see FIG. 20). At this time, the reception side performs user detection depending on whether it matches with a unique code assigned to the local device. In general, to reduce the probability that code patterns coincide with each other by accident due to noise or the like, at least one byte (8 bits) is often allocated to the user code. As the configuration of the communication frame, for example, various control codes are located before and after the user code and data (payload data) for a user are located following them (see FIG. 20).
As the conventional communication system carries out synchronous control and user detection by arranging preambles and a user code in a communication frame, however, there is a problem that the communication frame becomes redundant.